PERUN bc. richard dobrichovskýˇ A Lightning Plug-in for The Foundry’s NUKE Compositor Master’s Thesis Department of Computer Graphics and Interaction Faculty of Electrical Engineering Czech Technical University in Prague Supervisor: Ing. Jaroslav Sloup May 2016 Richard Dobˇrichovský: PERUN, A Lightning Plug-in for The Foundry’s NUKE Compositor, Master’s Thesis c May 2016 Set with the Classicthesis LATEXtemplate by André Miede. ACKNOWLEDGEMENTS I would like to thank my advisor, Ing. Jaroslav Sloup, for his construc- tive approach and advice. Also, I would like to thank my parents for their full support during my studies. DECLARATION I hereby declare that I have completed this thesis independently and that I have listed all the literature and publications used. Prohlašuji, že jsem pˇredloženou práci vypracoval samostatnˇea že jsem uvedl veškeré použité informaˇcní zdroje v souladu s Metod- ickým pokynem o dodržování etických princip ˚upˇripˇrípravˇevyso- koškolských závˇereˇcných prací. In Prague, the 27th of May, 2016 v ABSTRACT The goal of this thesis is to create a physically based lightning simu- lator for use within the VFX industry. The thesis studies the physics of real lightning creation and explores existing methods for its simu- lation, both visual and physically based. We use the Dielectric Break- down Model as our physically based method of choice and offer four variations of a conjugate gradient solver used within to solve systems of linear equations in order to calculate electric potential. Further, we also discuss the rendering of electrical bolts and scene illumination in- duced thereby. We implement our Perun lightning generator as both a standalone application and a plug-in for The Foundry’s Nuke com- positor. Finally, we demonstrate the use of the implemented plug-in to composite lightning into several shots and offer an insight to the process of doing so. Keywords: Perun, lightning, simulation, Dielectric Breakdown Model, NUKE, plug-in, video, VFX ABSTRAKT Cílem této práce je vytvoˇrení fyzikálnˇezaloženého simulátoru blesk ˚u jakožto efektu k použití ve filmové postprodukci. Tato práce nejprve zkoumá fyzikální podstatu blesk ˚ua prezentuje existující metody pro simulaci jejich tvaru, jak fyzikálnˇezaložené, tak visuální. Vybranou fyzikálnˇezaloženou metodou pro tuto práci je metoda pr ˚urazudiele- ktrikem; implementujeme ˇctyˇri varianty metody sdružených gradi- ent ˚upro ˇrešení systém ˚ulineárních rovnic k výpoˇctuelektrického po- tenciálu. Dále se v práci zabýváme renderováním elektrických výboj ˚u a osvˇetlováním scén, jichž jsou souˇcástí. Náš generátor blesk ˚uPe- run implementujeme jako samostatnou aplikaci i jako plug-in pro kompositor Nuke. Nakonec demonstrujeme užití tohoto plug-inu vložením vygenerovaných blesk ˚udo nˇekolika scén a objasˇnujeme postup k tomu zvolený. Klíˇcová slova: Perun, blesk, simulace, Dielectric Breakdown Model, NUKE, plug-in, video, VFX vii CONTENTS 1 introduction1 1.1 Motivation 1 1.2 Symbolics 1 1.3 Outline 2 2 physical properties of lightning3 2.1 Plasma 3 2.2 Downward Negative CG Lightning Formation 4 2.3 Other forms CG Lightning 5 3 shape generation9 3.1 Method Overview 9 3.2 The Dielectric Breakdown Model 15 3.3 Laplace’s Equation 17 3.4 Method of Conjugate Gradients 20 3.5 Adaptive Grid 21 3.6 Animation 23 3.7 Implementation 23 3.8 Benchmark 27 4 rendering 33 4.1 Bolt Shape Graph 33 4.2 Finding Branches 34 4.3 Drawing lines 35 4.4 Adding Glow 35 4.5 Scene Illumination 36 5 the foundry’s nuke plug-in creation 39 5.1 Overview 39 5.2 NDK C++ API 41 5.3 Implementation 43 5.4 Nuke Gizmos 44 5.5 Usability Testing 47 6 results 49 6.1 Sky 49 6.2 Sci-Fi 53 6.3 Jingle 54 7 discussion 55 8 conclusion 57 a implementation details 59 ix x contents a.1 Standalone Application 59 a.2 Nuke Plug-in 61 b enclosed cd 63 c list of symbols and acronyms 65 c.1 List of Symbols 65 c.2 List of Acronyms 66 references 67 LISTOFFIGURES Figure 1 A cloud-to-ground lightning strike. 4 Figure 2 Intracloud/intercloud lightning. 5 Figure 3 A visual breakdown of CG lightning forma- tion. 6 Figure 4 Rotoscoped lightning 10 Figure 5 First CGI lightning 10 Figure 6 Grid configuration for physically based meth- ods 13 Figure 7 Initial grid configuration for cloud-to-ground lightning 16 Figure 8 Grid configuration to illustrate matrix construc- tion 19 Figure 9 A generated lightning bolt and corresponding quadtree 22 Figure 10 Screenshots from nVidia Visual Profiler 26 Figure 11 Benchmark results plot 29 Figure 12 A sample render 34 Figure 13 A screenshot of Nuke 40 Figure 14 A screenshot of gizmo controls 45 Figure 15 Internal structure of gizmo 46 Figure 16 Comparison with lightning in the sky 50 Figure 17 A comparison of CGI and real lightning 51 Figure 18 A screenshot of compositing in Nuke 52 Figure 19 Use of the plug-in in a company jingle 54 LISTOFTABLES Table 1 Benchmark results per iteration for η = 1 28 Table 2 Benchmark results per iteration for η = 2 28 Table 3 Benchmark results per frame for η = 1 30 Table 4 Benchmark results per frame for η = 2 30 xi INTRODUCTION 1 1.1 motivation Although lightning is not a natural phenomenon one would experi- ence ever so often in real life, it would be hard to imagine a science- fiction film without it. In fact, on average on Earth, a lightning flash occurs more than 50 times per second, which is definitely not marginal. Therefore we find it a valid topic to look into in terms of computer simulation. In film/video production, effects such as lightning are normally added to the footage in post-production, as recording such effects together with the scene itself would be rather impractical, or even im- possible. Many effects found in science fiction films don’t even exist in reality, or would be harder and more expensive to create physically than as CGI. It is surprising to see, how many today’s films or series incorpo- rate some form of lightning: The Matrix Trilogy, Watchmen, Marvel’s Agents of S.H.I.E.L.D. or Star Wars Rebels just to name a few. These effects, either pre-rendered from 3D or generated directly in 2D, are blended into the footage during compositing, i.e. assembling multiple images to make a final image. Today, this is done digitally with a wide range of software, one of the most widely used in professional post-production being The Foundry’s NUKE. An effect that NUKE does not have under the hood is lightning. This might not be a crit- ically missing feature, as large studios will have their own, in-house developed effects plug-ins and a team of software developers to create effects tailored for the particular film in the making; smaller studios though have to rely on expensive 3rd-party plug-in collections, buy stock footage or use completely different software for compositing. Therefore we decided to create a dedicated lightning effect plug-in for NUKE to fill a small hole on the market. 1.2 symbolics The project was named Perun, after the ancient Slavic god of thunder and lightning and the highest god of the Slavic pantheon. The pre- vailing theory is that Perun’s name comes from the Slavic verb perti meaning “to beat”, although Téra offers a wider etymological view on the subject in [33]. 1 2 introduction Most polytheistic religions have their own god of thunder and light- ning—the Nordic Thor, the Celtic Taranis, the Greek Zeus or the Ro- man Jupiter—but Perun was chosen to reflect our Slavic roots. The symbol shown on the title page is called a thunder mark, a symbol of Perun’s. This particular symbol and several other varia- tions can be found engraved on chimneys or roof beams of many vil- lage houses in Eastern Slavic countries. They were believed to protect the buildings from lightning strikes. Perun is the central character in Havlíˇcek-Borovský’s satirical poem Kˇrest sv. Vladimíra [13], set in the time of tzar Vladimir I., who sentences Perun to death for declining to thunder on his nameday. The people are left without a god and therefore disobedient, so tzar Vladimir I. issues a competition for a new god. Though the poem is unfinished and does not explicitly end with Vladimir’s christening, it is believed to also reflect christianisation, which occurred under Vladimir’s reign in the region. 1.3 outline The thesis can be broken down into several distinct parts. In chapter 2 we start with a review of the physical phenomenon of lightning, what it is, what forms it can take and how it gets created in the atmosphere. In chapter 3 we continue with an overview of existing methods for lightning simulation in computer graphics and later present our chosen physically based method for shape generation in detail. Next, in chapter 4, we discuss the rendering of electrical bolts and scene illumination in relation to practical compositing. Chapter 5 first presents a survey of options available for Nuke plug- in creation and later explains the use of the NDK C++ API for the implementation of the Perun lightning plug-in for Nuke. An essential part of the thesis is showcasing the use of the plug- in in actual scenes—in chapter 6 we accompany the videos on the enclosed CD with a text description explaining how the scenes were composited. Finally we analyse our achievements, asses the shortcomings and offer several suggestions how the plug-in could be improved or ex- panded in chapter 7. PHYSICALPROPERTIESOFLIGHTNING 2 Although we do not experience lightning ever so often in our lives, on average, lightning strikes the surface of the Earth more than 50 times per second in over 200,000 thunderstorms a day [19].